UNDERSTANDING AND PROTECTING MIGRATING LANDBIRDS: STOPOVER NICHES, MIGRATION STRATEGIES, AND EFFECTIVE COLLISION-REDUCING GLASS

Abstract

North American bird populations have declined by more than 30%, and these declines have been especially pronounced for migratory landbirds, passerines and near-passerines that move seasonally between breeding and non-breeding ranges. Long-distance migration is hazardous, requiring birds to locate stopover habitat for rest and refueling within increasingly human-altered landscapes. The habitat that remains may be degraded, providing fewer resources and exposing birds to threats such as collisions with glass at stopover sites. Effective conservation of stopover sites depends on understanding how habitat availability, diet, and microhabitat selection differ among migrant landbirds, how these differences shape migratory decisions, and which bird-friendly glass treatments most effectively reduce window collisions during migration; however, this information is lacking. Therefore, in Chapter 1, I use models that maximize migration speed and minimize energy expenditure to assess how the availability of preferred land-cover types influences landbird migration strategies, as measured by stopover duration, arrival fuel load, and departure fuel load. Because most migratory landbirds use forests, and forest habitats are common across migratory routes, these species are generally presumed to follow a stopping–hopping migration strategy, characterized by many short stops after fuel stores are depleted. In contrast, shorebirds rely on limited coastal habitats en route and are thought to employ a staging–jumping strategy, characterized by fewer, longer stops before fuel stores are depleted. I parameterized the model with real-world data from four migrant landbird species and compared model outputs to fuel loads and stopover durations measured from captured birds to test the hypothesis that landbirds relying on rare wetland habitats optimize migration by adopting both a risk-averse strategy (greater arrival fuel loads) and a staging–jumping strategy (greater departure fuel loads and longer stopovers). The models predicted that species associated with rare wetland habitats adopt a staging strategy with fewer, longer stops, whereas forest-associated species adopt a hopping strategy with more frequent, shorter stops. Evidence from captured birds for a more staging-like strategy was strongest, though still limited, in Northern Waterthrush during pre-breeding migration. Nevertheless, greater arrival fuel loads for modeled and captured birds suggest that species using rare wetland stopover habitats benefit more from a risk-averse migration strategy than forest-associated migrants. Real-world results also indicate that the model assumption of time minimization during post-breeding migration is not supported, because captured birds had long stopovers and low departure fuel loads. These findings underscore the disproportionate importance of rare wetlands for migratory success and suggest that their loss could severely affect populations. In Chapter 2, I use tracking and genomics to further explore how ecological specialization relates to migration strategy. I tested the hypothesis that migrant landbirds do not share a broad generalized niche. Instead, I predict that species that breed in wetland and early successional landcover types have narrower niche position and breadth during migration than species that breed in forest cover types. I compared diet and microhabitat use of six species of migrant landbirds during pre and post breeding stopover at Powdermill Nature Reserve in southwestern Pennsylvania. I quantified diet with genetic analysis of fecal samples and microhabitat selection with individual tracking. Rather than all species, converging on a shared stopover niche during migration, I found differences in niche position and breadth that reflected forest vs wetland and early successional breeding habitat. Population vulnerability is a general consequence of constrained migratory niches; therefore, conservation strategies must move beyond generic cover metrics to maintain the structural and microclimatic diversity that sustain constrained niches. Recognizing that successful migration depends not only on strategies tailored to ecological requirements en route but also on avoiding human‑made hazards, Chapter 3 evaluates approaches to mitigating bird-window collisions. Specifically, I investigated how treatment design and glass optics interact to predict collision risk using a non-lethal binary-choice experiment with migrating birds. Given the scale of collision mortality and ongoing continent-wide declines in bird abundance, resolving interactions between treatment design and glass optics is critical for improving mitigation effectiveness and reducing population-level impacts. It is believed that making glass visible depends on creating dots or lines with sharply contrasting light–dark edges that birds can distinguish from the window’s optical background, which is determined by glass construction. Birds were captured at banding stations and released into a 24-m-long tunnel with two glass units at the far end: one patterned with a marker that birds might perceive as an obstacle and one unpatterned. Birds avoided high-contrast, conspicuous markers regardless of the optical background created by the window pane’s construction. In contrast, avoidance of more subtle markers, which are preferred aesthetically by most people, required specific glass constructions to achieve efficacy approaching that of high-contrast markers. Our results indicate that in collision-prone situations—where façades have high glass coverage, complexity, and abundant nearby vegetation or other attractants that bring birds into close proximity—designs should favor robust, highly visible markers because their detectability is less sensitive to background variability.